Conventional marine pipeline construction entails use of a barge to weld pipe segments together and then lay the assembled joints on the seafloor, using a pipe support stinger and maintaining proper tensioning of the pipe. An alternative to this "stove pipe" method is found in towing out from shore very long (one to ten mile) strings of welded pipe segments, then connecting these strings on-site to form the pipeline. This alternative method is referred to as a towed string method, regardless of whether the strings are towed on bottom or near the surface.
The joining of two long strings of pipe near the water's surface, normally associated with use of a towed string pipelay method, includes these cases: both string ends are initially on bottom and must be raised for joining; a previously laid string is initially on bottom and the other arriving string is floating, ready for joining; and a previously laid string is on bottom and the arriving string has been towed to location on bottom, but with the forward end near the surface, in a "nose up" fashion.
The conventional method for joining two such strings is to grip the pipe ends and align them based on a common horizontal pipe centerline axis. With the string ends in this position a weld is made or a mechanical connector installed. In order to prevent the suspended portions of the pipe strings from buckling during this operation, these suspended portions must be supported in the overbend by a stinger, davits or by buoys and in the sagbend by maintaining tension on the string ends. This is illustrated in FIG. 1.
Various problems are associated with this conventional method of joining two long pipe strings, e.g., large forces are required to deform the string ends into proper horizontal alignment; support stingers and/or buoys are difficult to control and subject to damage in rough offshore weather; and, support stingers and/or buoys are expensive and have a short useful life. Such problems make connection by this method risky and expensive.